1. Field of the Invention
The present invention relates to a gas pressure actuator, particularly to an air pressure actuator and a method for controlling the air pressure actuator.
2. Description of the Related Art
As an air pressure actuator, there has been one which was suggested by the inventors of the present invention and is shown in FIG. 1. Referring to FIG. 1, such an air pressure actuator comprises a guide shaft 14 extending in one axial direction with both ends thereof fixed on a pair of support members 18, and a slider 13 movable along the guide shaft 14. In fact, the slider 13 is a cylindrical hollow body which is so formed that it can cover up part of the guide shaft 13. In this way, a cylinder space is formed between the inner surface of the slider 13 and the outer periphery surface of the guide shaft 14. Practically, such a cylinder space is used as a pressure chamber. In more detail, the cylinder space has been divided (in its axial direction) into two pressure chambers 16A and 16B by virtue of a pressure receiving plate (partition wall) 17 fixed on the internal wall of the slider 13. Accordingly, both the pressure receiving plate 17 and the slider 13 are slidable along the guide shaft 14.
On both sides of the guide shaft 14 are provided a plurality of static pressure air bearings 12 arranged to be separated from one another at a predetermined interval in the circumferential direction. Practically, these static pressure air bearings 12 are connected with an air pressure source 10 through a regulator 11A. For this purpose, a plurality of air passages are formed in the guide shaft 14 and communicated with the static pressure air bearings 12. However, since each of the static pressure air bearings is a well-known bearing in the art, a detailed explanation as to the structure thereof will be omitted in this specification.
On both sides of the guide shaft 14 and with the two pressure chambers 16A, 16B are connected intake/exhaust systems for introducing a compressed air into the pressure chambers or for discharging the same therefrom. To ensure such an air instruction and discharge, a plurality of air passages, which are independent from the above air passages for use with the static pressure air bearings, are formed in the guide shaft 14, extending from both ends of the guide shaft to the pressure chambers 16A and 16B The intake/exhaust systems are respectively equipped with servo valves 22A and 22B so as to form a desired servo control. These servo valves 22A and 22B are all connected to the air pressure source 10 through a regulator 11B.
In this way, an air which is supplied from the air pressure source 10 and whose pressure has been properly regulated by the regulator 11A can be supplied to the static pressure air bearings 12. By virtue of the compressed air blown out of the static pressure air bearings 12, the slider 13 will float from the guide shaft 14, enabling the slider 13 to move with respect to the guide shaft 14 without touching it. For this reason, there would be no sliding resistance during the movement of the slider 13. Further, a position sensor 15 based on a linear scale or the like is used to detect the position of the slider 13, and to produce an electric signal representing the slider""s position. The detected position signal fed from the position sensor 15 is then fed to a controlling and computing device 20.
The controlling and computing device 20 performs control and computation based on the inputted position information and outputs position instruction signals to servo amplifiers 21A and 21B. At this time, the instruction values to be fed to the servo amplifiers 21A and 21B are those having the same absolute values but opposite signs.
The servo valves 22A and 22B receive the supply of a compressed air which has been regulated by the regulator 11B to an appropriate pressure, while the flow rate of an air flowing through each of these valves can be changed depending on the position of a spool within each of the servo valves 22A and 22B. Air flows which have passed through the servo valves 22A and 22B are supplied to the pressure chambers 16A and 16B formed within slider 13. As a result, a pressure difference occurs between the pressure chamber 16A and the pressure chamber 16B, and such a pressure difference acts on the pressure receiving plate 17 provided on the internal wall of the slider 13, causing the slider 13 to move in one of the two directions.
Since the air pressure actuator described above can control a large amount of output with a compact structure, it has been expected to be used as an actuator for performing a positioning between any two points. However, when performing a continuous positioning, such an air pressure actuator has been found to be difficult in performing a stabilized control, because a dynamic characteristic change and the like based on the position of the pressure receiving plate are non-linear. Consequently, it is difficult to obtain an effective long stroke with respect to a mechanical stroke of the slider. This is because whenever the position of the pressure receiving plate is changed within the pressure chambers, the pressures within the pressure chambers will also change, hence bringing about an undesired influence to a stabilized control.
Accordingly, it is an object of the present invention to provide an improved gas pressure actuator based on a gas pressure actuator whose slider is driven by a gas pressure using two servo valves, which is characterized in that its dynamic characteristic change depending on the position of the slider can be compensated, so as to effect a stabilized control of the slider within the stroke thereof. Further, it is another object of the invention to provide an improved method for controlling the gas pressure actuator described above.
A control method according to the present invention can be suitably applied to a gas pressure actuator described hereunder. The gas pressure actuator includes a guide shaft and a slider movable along the guide shaft, and a pressure receiving plate provided on one of the guide shaft and the slider to form a cylinder chamber between the outer surface of the guide shaft and the internal surface of the slider and to define the cylinder chamber into two pressure chambers arranged side by side in the slider moving direction. The gas pressure actuator is constructed in a manner such that a compressed gas is introduced into or discharged from the two respective pressure chambers by way of servo valves, so as to use a pressure difference between the two pressure chambers to drive the slider. Further, the gas pressure actuator includes a position sensor for detecting the position of the slider, two servo amplifiers for controlling the servo valves, and a controlling and computing device for receiving a position detection signal fed from the position sensor and for producing position instruction values to the two servo amplifiers.
According to an aspect of the present invention, the method comprises the steps of: performing a computation on each of the position instruction values to be fed to the two servo amplifiers, so as to compensate for a pressure change which has occurred in each of the pressure chambers due to a change in the position of the pressure receiving plate in the cylinder chamber; and producing position instruction values to the two servo amplifiers.
A gas pressure actuator according to the present invention is characterized by incorporating an improved controlling and computing device which performs the following steps. Namely, the controlling and computing device performs the steps of: differentiating a slider position represented by a detected position signal, and calculating the velocity of the slider, meanwhile differentiating the calculated velocity so as to calculate an acceleration; using a slider target position, said slider position, said velocity and said acceleration to calculate position instruction values to be fed to the two servo amplifiers; performing a computation on the respectively calculated position instruction values, so as to compensate for a pressure change which has occurred in each of the pressure chambers due to a change in the position of the pressure receiving plate in the cylinder chamber; and producing the respectively compensated position instruction values to the two servo amplifiers.